The invention relates to a method of recording information which can be read optically while using a recording element which is exposed to modulated laser light and which comprises a substrate plate and a recording double layer of a synthetic resin provided on one side, of which a first layer of synthetic resin which engages the substrate, has a comparatively high coefficient of thermal expansion and a glass transition temperature which is below room temperature and a second layer of synthetic resin of which which is bonded to the first layer has a comparatively low coefficient of thermal expansion and a glass transition temperature which is above room temperature.
Such a method is described in European Patent Application no. 0136070. According to the method known from this Application information is recorded, read and erased again, while using the above-mentioned recording element, reversible process.
Upon recording information the recording element on the side of the recording double layer is exposed to laser light which has been pulsated in accordance with the digitalised information to be recorded. As a result of this exposure the first layer of synthetic resin--which will hereinafter also be referred to as the lower layer or expansion layer--is heated in the exposed places, the material expanding considerably. The heating is caused by the expansion layer absorbing the laser light used. The second layer of synthetic resin--hereinafter also referred to as the top layer or retention layer--is slightly heated in the exposed places because this layer is transparent or substantially transparent to the laser light used. This slight heating is caused by thermal conductivity from the heated lower layer or by a slight direct absorption or the laser light by the top layer.
The temperature of the top layer increases to slightly above the glass transition temperature at which temperature the material is rubbery. As a result of the expansion of the lower layer the top layer is forced upwards and a bump is formed. Upon cooling, the top layer rapidly gives off the small quantity of thermal energy in which the temperature comes below the glass transition point and the top layer obtains a rigid structure. The lower layer is still in a heated and hence expanded state. Upon further cooling the lower layer cannot further shrink because it is fixed to the top layer bonded thereto. The bump hence does not disappear. A tensile force is exerted on the deformed top layer by the lower layer. The recorded information bit, in this case the bump, can be read by means of laser light on the basis of phase differences.
In the erasing process laser light is used of a different wavelength which is absorbed by the top layer and is passed by the lower layer. The top layer is heated to above the transition point. As a result of the tensile forces mentioned hereinbefore exerted by the lower layer the rubbery, the heated top layer is reverted to its original position and the bump is erased.
In order to realise that the lower layer at a first wavelength is absorbing and at a second wavelength is transparent and the top layer just shows the reverse, namely transparent at the first wavelength and absorbing to laser light of the second wavelength, dyes are used in the lower layer and in the top layer which have essentially deviating absorption characteristics.
From test series performed by the Applicants it has been found that the known method described hereinbefore has for its disadvantage that upon recording bits (bump) of different longitudinal dimensions the width dimensions of the bits vary. When the length of the bits increases, the width also increases. The increase in width is not proportional to the longitudinal increase. Bit length modulation is an important subject, for example, a bit length modulation is necessary to record information modulated according to the EFM (eight out of fourteen modulation) system. In the very known optical audio discs (compact disc-.TM.-) the sound information is recorded according to the EFM system. Bits are used having lengths of 0.9 .mu.m, 1.2 .mu.m, 1.5 .mu.m, 1.8 .mu.m, 2.1 .mu.m, 2.4 .mu.m, 2.7 .mu.m and 3.0 .mu.m.
Since the bits have different width dimensions, the width of the information track which usually is piral-like, is not constant. As a result of this the space between the turns of the spiral-like track varies whimsically. Consequently, the above-mentioned known method is not suitable for recording EFM modulated information. The information density is not optimal since it is determined in particular by the width of the track and the associated width of the track pitch.